1. Field of the Invention
The invention relates to a powder-metallurgy-produced, essentially titanium-free, nickel-containing maraging steel die block article with especially good properties for metal die casting dies and other hot work tooling components and to a method for producing the same.
2. Discussion of the Prior Art
Dies used for die casting alloys of aluminum, magnesium, and other metals require steels that have good strength and toughness at ambient and elevated temperatures and high resistance to thermal fatigue. They also require steels that can be readily machined and that can be heat treated after machining with minimum difficulty and distortion. Currently, most die casting die components and other hot work tooling components are machined from die blocks that are cut from hot worked slabs or forgings.
The high-nickel, titanium-bearing maraging steels are excellent materials for use in die casting applications as all of the machining may be performed on the die blocks prior to age hardening. In addition, these steels in the age-hardened condition exhibit high strength in combination with high impact toughness and good thermal fatigue resistance, which promote long service life. Current high-nickel, titanium-bearing maraging steels have a serious drawback, however, in that their solidification characteristics result in significant segregation of the alloying elements during casting. This segregation can be detrimental to the properties of the steel, and especially to thermal fatigue resistance. In addition, this segregation inhibits the potential use of these steels in die casting dies that are cast to near-net-shape. When produced in ingot form, the high-nickel, titanium-bearing maraging steels are typically vacuum arc remelted to minimize segregation in the final product. This substantially increases the cost of the articles made from them.
Attempts have been made to minimize the segregation problems in high-nickel, titanium-bearing maraging steels by processing them by hot isostatic compaction of elemental or prealloyed powders made by conventional practices such as rotating electrode or argon gas atomization. However, the ductility and impact toughness of the as-compacted, powder-metallurgy-produced materials have generally been less than the ductility and impact toughness of conventionally-produced, ingot-cast material in the wrought condition. This appears to result from the segregation of the titanium and the formation of titanium-rich carbides and other compounds at the powder particle boundaries of the consolidated article made from the powder. It has been determined that hot plastic deformation can improve the impact toughness and tensile ductility of the high-nickel, titanium-bearing, powder-metallurgy-produced maraging steels to levels approaching those of conventionally-produced materials. However, the presence of the titanium-rich compounds in these materials still adversely affects their machinability. Furthermore, the amount of hot work needed to improve their properties is difficult to achieve at the center of large dies or die blocks where the extent of hot deformation is typically lower and less uniform than in other areas of the cross section. Thus, up to now there appear to be no fully practical methods for the powder metallurgy production of high-nickel maraging steels for die casting die blocks and related articles.
In work on the development of improved die casting die steels and articles made therefrom in accordance with the invention, it has been discovered that a more economical nickel-containing maraging steel with substantially better properties for metal die casting applications can be produced by gas atomization and hot isostatic compaction of essentially titanium-free, nickel-containing maraging steel powders. The prior art indicates that the elimination of titanium from nickel-containing maraging steels would significantly degrade their strength and age-hardening response. However, contrary to these prior art teachings the essentially titanium-free, nickel-containing maraging steel produced in accordance with this invention has unexpectedly good properties, and exhibits tensile properties, hardening response during aging, and thermal fatigue resistance which are substantially superior to those of conventionally-produced, titanium-bearing, nickel-containing maraging steels and articles made therefrom. In addition, the essentially titanium-free, nickel-containing maraging steel article produced in accordance with this invention exhibits substantially better machinability in combination with the above-mentioned properties than conventionally-produced, titanium-bearing, nickel-containing maraging steel articles. Also, it has been discovered that by adding a controlled amount of niobium to the powder-metallurgy-produced, essentially titanium-free, nickel-containing maraging steel article of the invention, a further substantial improvement in thermal fatigue resistance can be obtained without a loss in mechanical properties.